The present invention relates to a process for preventing back flow of metal between the moving mold belts of a caterpillar type track mold and the nozzle for feeding molten metal to the mold.
One of the most difficult problems in continuous casting, in particular casting ferrous and non-ferrous metals, arises from the design of the feeder nozzle by means of which the molten metal is introduced into the mold between the moving mold belts of a caterpillar track type mold. In the case of a caterpillar track type casting machine thin strip, that is, 20 mm thick is cast. Accordingly the nozzle must be of relatively small dimension especially in the region of the nozzle's mouthpiece.
One damage risk factor associated with the nozzle is the very high temperature of the metal flowing through it. There are only a few materials which are able to resist erosion or dissolution in the metal. Graphite is one of the few materials which can meet the requirements. Graphite, however suffers from the disadvantage that it is highly thermal conductive, with the result that the metal has a tendency to solidify in the nozzle.
Another suitable refractory material used widely in casting aluminum is a mixture of 30% diatomaceous earth (almost pure silica in the form of microscopic cells), 30% asbestos fibers, 20% sodium silicate (dry mixture) and 20% chalk (to form calcium silicate). Steel on the other hand is usually cast by employing a nozzle of pure ZrO.sub.2 or ZrSiO.sub.4.
The nozzle must not only be able to resist the thermal stresses due to the high temperature of the metal being cast, but also must withstand the resultant chemical attack and mechanical effects arising from fluctuating movement of the belts and bending of the nozzle due to the relatively large weight of the melt flowing through it. The bending or sagging of the nozzle leads to frictional rubbing of the nozzle on the belts and with that destruction of the nozzle itself.
Another problem encountered is back flow of metal, that is, molten metal from the nozzle flowing backwards over it. The molten metal emerging from the nozzle forms, in the region between the nozzle outlet and the point of first contact with the upper moving mold belt, a radius of curvature which depends essentially on the surface tension of the metal, the metallostatic pressure at which the metal leaves the nozzle, and the speed of the moving mold belts. Due to premature solidification of the metal a build up of metal can occur which causes the metal to flow back behind the tip of the nozzle. This back flow is very undesirable as it interrupts the continuous casting process very markedly and hinders proper coordination of the nozzle and the casting machine.
A known method for avoiding the above mentioned problems is revealed in the Swiss patent application No. 3019/83 wherein an air cushion is formed in a space between the nozzle and the moving mold belts. By means of the air cushion a radius of curvature on the molten metal between the outlet of the nozzle and the point of contact of the melt with the upper mold belt is influenced. The foregoing method is very effective but requires a particular design of nozzle body and exact knowledge of the force of the air stream to be supplied and the amount of pressure in the air cushion formed.
Accordingly, it is the principal object of the present invention to develop a process to prevent the back flow of metal at the nozzle.